1990 Mazda Miata white smoke at pipe

White smoke is usually caused by coolant in the ignition system (blown head gasket or water in the gas tank due to condensation). You should do a compression test to check your head gasket. For future reference white smoke is water/coolant; Blue smoke is oil; Black smoke is flooding or too much gas.

It is very rare but sometimes a brake fluid leak at the master cylinder can allow the vaccuum operated brake booster to suck in fluid into the intake manifold, causing white smoke.

I suggest starting with checking your head gasket.